Due to improvement in luminance of a white LED (Light Emitting Diode), a light source device employing a white LED has been used for a lighting apparatus and a liquid crystal TV. Such a light source device can be classified into a direct backlight device or an edge light backlight device.
The direct backlight device is mainly used for a liquid crystal TV and has a configuration in which a surface light source device is provided on a rear surface of a liquid crystal panel. The surface light source device is configured such that a plurality of white LEDs are mounted on a planar mounting substrate in matrix.
On the other hand, the edge light backlight device has a configuration in which a linear light source is provided on a side surface of a light guide plate that is provided on a rear surface of a liquid crystal panel. The linear light source is configured such that a plurality of white LEDs are mounted on a linear long-narrow mounting substrate along a longitudinal direction of the mounting substrate. Due to easiness in reducing a thickness of the edge light backlight device, the edge light backlight device makes it possible to provide a thin liquid crystal display TV and therefore has been attracting much attention.
The edge light backlight device can be further classified into two types. The first type employs an LED part called a side-view light emitting type LED. In the first type, an LED package side surface (a side surface of a reflector) serves as a mounting surface and light is emitted along a direction parallel to the mounting surface. The first type has been used in backlight devices or the like for which a luminance requirement is not very severe. An example of such backlight devices is a backlight device for a small-size to medium-size liquid crystal display device. Patent Literature 1 discloses a display device employing the first type. The second type employs an LED part called a top-view light emitting type LED. In the second type, an LED package bottom surface serves as a mounting surface, and light is emitted in a direction perpendicular to the mounting surface. Patent Literature 2 discloses a liquid crystal display device employing the second type.
In the first type employing the side-view light emitting type LED, an area of the LED package side surface serving as the mounting surface needs to be sufficiently wide so that stability is secured in a product after the mounting. Due to this, the first type has a reflector whose height is greater than that of the second type, and accordingly has a greater light loss in the reflector as compared to the second type.
On the other hand, in the second type employing the top-view light emitting type LED, there is a concern about efficiency in bonding of the LED package bottom surface and a light guide plate, because the LED package bottom surface serves as the mounting surface. However, it is possible to have a reflector whose height is smaller than that of the first type, and therefore the second type makes it possible to reduce light loss in the reflector.
Further, in a backlight device for a liquid crystal TV, high-luminance LEDs are employed and mounted in larger numbers. This increases a quantity of heat produced when the backlight device is driven. Due to this, LED packages are required to have higher levels of heat dissipation characteristics and heat resistance properties.
Regarding the side-view light emitting type LEDs, many products are formed by insert molding of a lead frame. In such a side-view light emitting type LED, the lead frame serves as a main heat dissipation path. Accordingly, the heat dissipation characteristics of an LED package depend on a length of a portion of the lead frame as measured down to the mounting surface and on a width and a thickness of the lead frame. In the side-view light emitting type LED, because the side surface of the reflector serves as a mounting surface, the length of a portion of the lead frame as measured down to the mounting surface is longer than that in the top-view light emitting type LED. Further, because the lead frame is to be bent, there is a limit in an extent to which the width and the thickness of the lead frame can be increased. This makes it difficult to obtain desired heat dissipation characteristics. Furthermore, in order to increase an area of an external terminal section of the lead frame (i.e., a surface portion bonded with the mounting surface), the area of the side surface of the reflector needs to be increased. This will cause an increase in light loss in the reflector.
On the other hand, in the top-view light emitting type LED, the thickness and the width of the lead frame cannot be easily increased because the lead frame is to be bent. However, because the LED package bottom surface serves as the mounting surface, the length of the portion of the lead frame as measured down to the mounting surface can be set shorter than that in the side-view light emitting type LED. Due to this, even if an area of an external terminal section of the lead frame (i.e., a surface portion bonded with the mounting surface) is increased, the increase will not have a negative effect on optical characteristics.
Therefore, for a large-size backlight device (edge light backlight device) of a liquid crystal TV or the like, the backlight device employing the top-view light emitting type LED is used.